Systems for interrogating and identifying an object are equipped with an antenna having a given angle aperture making it possible to interrogate in a direction (emit a directional beam) and in a controlled angle sector. These interrogator or interrogator/responder systems on account of the directivity of their antenna do not generally make it possible to carry out passive listening. Indeed, the angle sector over which they operate is limited. Moreover, the low emission rate of unsolicited responders (better known as “squitters” (˜1 Hz)), allied with the use of traditional interrogator/responder or IFF (Identification Friend and Foe) antennas (sum pathway Σ/difference pathway Δ with an aperture of ˜10°) implies that, to chart the aerial situation, an IFF interrogator customarily used in the prior art must perform numerous antenna revolutions. The response of the “squitter” will be received only when it is situated in a precise angle sector and the time for observing its response will therefore be limited (duration for which the response is observed).
Current antenna systems (Σ/Δ antenna patterns necessary for the sidelobe suppression processing, RSLS for short) used by IFF interrogators are therefore completely unsuitable for the squitters' listening function which requires an omnidirectional antenna.
Various solutions are currently known in the prior art. Various examples are given in relation to FIGS. 1, 2 and 3 which comprise in a common manner a digital core 1, a signal processing device, having notably the function of eliminating the responses of the antenna sidelobes or RSLS, for decoding the information of the signal received by the system, a reception pathway 4 corresponding to the sum pattern of the antenna, a reception pathway 5 corresponding to the difference pattern of the antenna 6 with which the responder interrogator is equipped.
A first solution for responding to the passive listening requirement is illustrated in FIG. 1. It consists in adding a reception pathway 7 and an omnidirectional antenna 8 dedicated to the listening function, as well as a device 3 for processing and decoding the information received on this extra pathway. The addition of such an antenna and of a dedicated reception pathway gives rise to the presence of a dedicated digital processing pathway. These additions engender problems of complexity of installation, costs (consumption of the FPGA digital processing devices, etc.)
A second solution illustrated in FIG. 2 relies on the addition of a switching device 9 making it possible to use an omnidirectional antenna 10 on the sum reception pathway of the antenna customarily used. Such a device gives rise to the addition of an omnidirectional antenna and an increase in the complexity of the radiofrequency function and of installing the assembly.
A third solution known to the Applicant is illustrated in FIG. 3. It consists in adding an extra processing chain 11 on the pathway of the sum pattern or Δ. In the example of FIG. 3, the addition is done on the difference reception pathway or Δ. This leads to an increase in the complexity (with 2 decoding pathways) of the extra processing module on the pathway as well as limitations related to the “blind” zone on the axis of the antenna Δ.